1. Field of the Invention
The present invention relates generally to inflatable medical balloons, and more particularly, to a method and mold for fabricating thin cone balloons. The mold has a unique configuration with inverted, partial conical molding surfaces, to provide balloons which provide enhanced performance and patient safety in terms of a lower profile and improved rewrapping characteristics.
2. Description of the Prior Art
Inflatable medical balloons associated with balloon catheters are well known in the art, and are commonly used in, for example, angioplasty procedures. The balloon includes a cylindrical main body, tapered transition regions, and connecting portions where the balloon is attached to a catheter. The balloon is typically advanced in a collapsed state into an artery or other passage of a patient to a region of stenosis and thereafter the balloon is inflated. The balloon must have a small deflated profile to enable advancement through tortuous paths, without causing damage to the surrounding inside surface of the blood vessel. After the inflation procedure is completed, the balloon must be deflated and withdrawn from the patient.
The process of fabricating such balloons generally comprises the steps of placing an extruded cylindrical tubular parison made of a drawable polymer having a specified diameter and wall thickness into a mold, heating the parison in its amorphous state to a blowing temperature, and expanding the parison within the mold while simultaneously drawing the parison longitudinally. A common material used in this procedure is polyethylene terephthalate ("PET"). Parisons of amorphous PET can be drawn and expanded to a final wall thickness of less than 0.001 inch in the main body of the balloon with wall thicknesses that are larger in the tapered transition regions disposed on opposite sides of the main body portion. The greater thickness in the transition regions is undesirable, as the small deflated profile of the balloon necessary to facilitate insertion through an artery or passageway requires that the balloon be folded and wrapped around the catheter. Unfortunately, during such folding, bumps or protrusions can occur in the transition regions. The increased thickness in the transition regions can cause these distorted areas to be relatively stiff and resistant to passing through narrow stenosis. Large diameter, high pressure angioplasty balloon catheters, in which the diameter of the balloon is in the range of from about 5 to 12 millimeters, are particular affected. The prior art has addressed these problems by reducing the transition wall thickness through various heating, blowing and drawing steps during the fabrication process.
U.S. Pat. No. 5,334,146 to Ozasa ("the '146 Patent") discloses an inflatable medical balloon fabricated from a polymer and comprising a cylindrical portion or main body region of a substantially uniform diameter, tapered portions at opposite sides of the cylindrical portion and connecting portions at the ends of the tapered portions, where the wall thicknesses of the middle parts of the tapered portions are equal to or smaller than 1.2 times the wall thickness of the cylindrical portion. The method of manufacturing the balloon comprises the steps of: forming a tubular parison made of a drawable polymer; heating the parison at a temperature in the range from the second-order transition temperature to the first-order transition temperature of the polymer used; stretching the parison in the direction of its longitudinal axis and inflating it radially while heated; cooling the stretched and inflated parison below the second-order transition temperature of the polymer; and thereafter deflating the parison. A crude balloon is thereby formed having a cylindrical portion of a substantially uniform desired diameter and wall thickness, with tapered portions having wall thicknesses greater than the desired final thicknesses. The tapered portions of the crude balloon are then redrawn to reduce their respective wall thicknesses to the final desired thicknesses by longitudinally stretching the tapered portions.
U.S. Pat. No. 4,963,313 to Noddin et al. ("the '313 Patent") discloses an inflatable medical balloon formed by fabricating a tubular preform having a tapered region at the end of the portion where the main body of the balloon will form to enable the corresponding transition section of the blown balloon to have a separately controllable thickness profile. The method of fabrication includes providing a parison of a selected resin having a wall thickness and diameter suitable for being formed into a balloon, and selectively heating a defined region of the parison at one or both ends of the portion of the parison from which the balloon is to be formed to a drawing temperature. Tension is then applied in opposite directions to respective ends of the heated region to draw the heated region to a smaller diameter, thereby providing a preform having a tapered, relatively small diameter region of material that has substantially no crystallization or molecular orientation at the end portion of the parison. The parison is then heated to a blowing temperature and the balloon is formed by drawing and blowing the entire parison, including the tapered regions. The formed balloon is then mounted on a catheter in a conventional manner. The '313 Patent teaches forming balloons having a wall thickness in the respective tapered regions that is substantially the same as the thickness in the main body. It also states that the wall thicknesses in the respective tapered regions may be reduced by a further drawing step on the defined region to achieve a wall thickness in the tapered regions which is less than that of the main body.
None of the above references is enabling as to an effective method for reducing the wall thickness in the tapered transition regions of the balloon to less than that of the main body using a simple forming process. The '146 Patent discloses a balloon having a wall thickness in the tapered region which is greater than that of the main body. Although the '313 Patent states that the wall thickness in the tapered region may be reduced to less than that of the main body by a further drawing step, it does not specifically describe how or with what type of fixtures or molds such is to be implemented.
U.S. Pat. No. 5,358,486 to Saab ("the '486 Patent") discloses a balloon for a dilatation catheter, which is fabricated by forming the balloon in a plurality of separate, very thin layers to provide a more uniform wall thickness in the tapered transition regions. The balloon includes an inner layer which defines the cylindrical main body, tapered transition regions and connecting portions. The main body and tapered transition regions are built up from a plurality of layers. The layers of the balloon are formed successively from the outermost layer to the innermost layer. Each successive layer is co-molded within the previously formed layer(s), and trimmed to size in the tapered transition regions to be slightly longer than the next adjacent inner layer. The procedure is repeated until the desired wall thickness is achieved. The final, innermost layer is not trimmed in the conical transition regions, thereby defining the complete balloon. While effective, this method requires multiple forming and trimming steps to fabricate the balloon.
Accordingly, there exists a need for a new method of fabrication and mold for forming an inflatable balloon having a reduced wall thickness in the tapered transition regions for superior insertion characteristics and increased patient safety.